Although naked DNA vaccines delivered by intramuscular or intradermal routes prime both antibody and T-cell responses, the level of the responses are often quite low. The low responses have been thought to be the result of inefficient plasmid delivery, both to the physiological site of interest and the rate of uptake by the cell.
In vivo electroporation techniques are available to assist in the delivery of DNA vaccines to subjects. Often, the DNA vaccination using electroporation techniques utilizes constant voltage electroporators, and such instruments do not consider tissue resistance and thus may give rise to non-optimal plasmid expression, inflammation and possible tissue damage.
In the area of human immunodeficiency virus (HIV) vaccines, the discovery of a prophylactic HIV-1 vaccine capable of eliciting high titered neutralizing antibodies and broad T-cell immunity has been elusive. Given that both arms of the immune system have been found to be integral in the control of HIV-1 disease progression (Pantaleo and Fauci, 1996; Soudeyns and Pantaleo, 1999), their induction remain core requisites of many vaccine research and development efforts. Previous reports have discussed results of studies with mice, rabbits and nonhuman primates showing induction of antibodies and T-cell responses using a polyvalent HIV-1 DNA prime/protein boost vaccine strategy (Cristillo et al., 2006; Pal et al., 2006a; Pal et al., 2005; Pal et al., 2006b). From these studies, as well as findings reported by others (Amara et al., 2001; Barouch and Letvin, 2000; Earl et al., 2001; Franchini et al., 2004; Gomez-Roman and Robert-Guroff, 2003; Reyes-Sandoval et al., 2004; Santra et al., 2004; Sumida et al., 2004; Zhao et al., 2003), DNA immunization delivered via intramuscular or intradermal route of administration, was shown to elicit measureable, but weak antibody and T-cell responses; and, thus, requiring boosting immunizations. Using an adjuvant-formulated recombinant protein, it was shown that both humoral and cellular immune responses that were primed by DNA vaccination could be further augmented (Cristillo et al., 2006; Pal et al., 2006a; Pal et al., 2005; Pal et al., 2006b).
In order to increase immunological priming by DNA immunization, several parameters have been investigated that include DNA dosage and scheduling, co-administration of adjuvant and immunomodulatory agent, and varying immunization strategies and routes of delivery. Such studies have highlighted the utility of electroporation technology in increasing expression of plasmid DNA and augmenting humoral and cellular priming using HIV-1, hepatitis B virus, smallpox, and tuberculosis vaccines. Although several electroporation platforms tested thus far have generated encouraging data, these studies have used constant voltage electroporators to facilitate plasmid delivery, which results in less than immunoprotective outcomes.
Similar issues have been described for cancer therapies. Cancer is a leading cause of death in the world with an estimated 9 million people dying from cancer in 2015, and 11.4 million dying in 2030 [World Health Organization]. Many of these deaths can be avoided with early detection and treatment, while over 40% of cancers are preventable. The overriding need for preventative and therapeutic remedies for the copious forms of cancer is daunting. However, new diagnostic technologies and screening methods as well as new and more effective therapeutic agents are available that altogether are reducing mortality for several cancers [www.cancer.gov]. Furthermore, the recent success of DNA vaccines based on the greater knowledge of tumor immunology in animal models has given hope to the field (e.g. melanoma, prostate cancer).
Although inflammation has been shown to promote oncogenesis (A. Mantovani, P. Romero, A. K. Palucka, F. M. Marincola, Tumour immunity: effector response to tumour and role of the microenvironment Lancet 371, (2008) 771-783), the immune system may also be utilized in the fight against cancer. The field of tumor immunology has evolved since the identification of the first tumor antigen in a human melanoma cell line in 1991, namely “MAGE” (melanoma Ag), that elicited a cytotoxic T-lymphocyte (CTL) response. Furthermore, it was discovered that this antigen is encoded by a normal, non-mutated gene that is activated in many melanomas and in some other cancers but remains silent in normal tissues. The presence of antigens on the surface of tumor cells recognized by cytotoxic and T helper lymphocytes is essential for effective immune responses and for the development of specific cancer vaccines. Unfortunately, the development of cancer vaccines for established cancers has not proven effective. The use of antibodies to tumor antigens has had limited success in treating specific types of cancer, possibly due to the action of regulatory T-cells. Regulatory T-cells normally function to restrain the activity of the immune response and have been associated with prevention of antitumor immunity (M. Beyer, J. L. Schultze, Immunoregulatory T cells: role and potential as a target in malignancy Curr. Oncol. Rep. 10, (2008) 130-136). Mechanisms of tolerance and immunoescape have also limited the clinical outcome of cancer vaccination, including DNA vaccines. In order to overcome these drawbacks and augment the immune response several lines of action are required. Firstly, it is important to identify the tumor antigens that are to be targeted. Secondly, the desired immune response needs to be defined and the optimal vaccine engineered. Finally, efficient delivery of the vaccine is essential for success (Bodles-Brakhop A M, Draghia-Akli R. DNA vaccination and gene therapy: optimization and delivery for cancer therapy. Expert Rev Vaccines. 2008 September; 7(7):1085-101).
Curcio et. al. Cancer Gene Therapy (2007) entitled “DNA immunization using constant-current electroporation affords long-term protection from autochthonous mammary carcinomas in cancer-prone transgenic mice,” which is noted as being published on-line in early November, 2007, discusses the use of electroporation facilitated DNA delivery methods and generating antibodies in mice against ErbB-2, Her-2/neu.
There still remains a need for a delivery method for DNA plasmids that yields antigen expression levels that can elicit a strong immune response and also generate neutralizing antibodies.